Apparatus and method for reducing noise in seismic data

ABSTRACT

A method of acquiring seismic data. The method includes receiving seismic signals at one or more sensors; sampling the received seismic signals into a plurality of samples, each sample having a same bit length; arranging the samples into a plurality of packets; compressing the plurality of packets by varying a time interval between transmissions of at least some of the packets; and transmitting the compressed packets.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from U.S. Provisional Application No.60/887,788, filed Feb. 1, 2007, which is incorporated herein byreference in its entirety.

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

This disclosure relates to the acquisition of seismic data using seismicspreads.

2. Background of the Art

Seismic surveys are conducted to map subsurface structures, which mapsare used to locate and develop oil and gas reservoirs. On land, seismicsurveys are conducted by deploying a large array of seismic sensors overselected geographical regions. Typically, these arrays can cover severalsquare kilometers (for example 100 square kilometers) of a geographicalarea and may include in excess of 30,000 seismic sensors (also referredto as receivers) placed in the ground and arranged in the form of agrid. The receivers are typically geophones and/or accelerometers forland operations. Three-axis accelerometers are often used as receivers.

An energy source, such as an explosive charge (buried dynamite, forexample) or a mobile vibratory source is typically used at selectedlocations in the array to generate acoustic waves or signals (alsoreferred to as acoustic energy) that propagate through the subsurfacestructures of the earth. The generated acoustic waves reflect atsubsurface formation discontinuities, such as boundaries associated withlayers of different rock types, salt domes and oil and gas reservoirs.These reflections are sensed at the surface by the seismic sensors inthe array. Sensors are typically grouped in small numbers and each groupis connected to a separate data acquisition unit (also referred to as arecording unit, or a field service unit). Each data acquisition unitreceives the signals from its associated sensors, samples the signals,digitizes the samples, stores the digitized samples, arranges thedigitized samples into packets and transmits such packets to a centralcontrol unit (also referred to as a central recording unit), eitherdirectly or via one or more intermediate units and/or repeaters.

The recorders may transmit the packets via cables or wirelessly to thecentral control unit, which may be on a mobile unit, such as a truck orat another remote location. The central control unit typically processesthe data (at least partially) received from the data acquisition units,stores the processed data for later processing and may send theprocessed data to another remote unit for further processing of thedata. A two or three-dimensional map (also referred to as a seismicimage) of the subsurface structures is generated by processing of thedata received from the central control unit.

Offshore seismic data acquisition systems typically utilize a compressedair source, such as an air-gun, as the seismic energy source, which isactivated at selected locations a few meters (often 5-6 meters) belowthe water surface while being towed by a vessel. The receivers aredeployed either in streamer cables that are towed by the vessel carryingthe source or are deployed at the ocean bottom in the ocean-bottomcables. Hydrophones are typically used as the receivers for offshoreapplications.

In a seismic spread, each recorder transmits a large number of packets.Typically, each packet may contain an “epilog,” a payload and a “prolog”that includes a large number samples or words (for example, about 500),each sample having a prescribed number of bits (for example, twelve bitsor twenty-four bits, etc). Often, the useful portion of the wordincludes less than the total number of available bits. Therefore, someor many samples in a packet may occupy bit spaces that contain onlyleading sign bits. The leading sign bits are simply “ones” or “zeros.”Accordingly, there is a need for an improved method and apparatus forpreparing, storing and transmitting packets.

Also, it is known that the recorder units in seismic spreads experiencecoherent noise. Coherent noise is periodic in nature. It is typicallyundesirable seismic energy that shows a consistent phase from trace totrace, such as ground roll and multiples. Coherent noise can occur dueto several different factors, such as: the presence of a common modeinduction at the receiver input due to data transmission; and electronicswitching in the intrinsic circuits coupled into the receiver by variousmethods, such as telemetric transmitters radiating energy, limited powersupply noise rejection, common circuit elements such as power suppliesor ground planes, and high energy computation bursts such as thosepresent during data transfer or intrinsic math functions. Therefore,there is a need for a method and an apparatus that may reduce thecoherent noise in seismic data acquisition systems.

SUMMARY OF THE INVENTION

The disclosure herein in one aspect provides a method of acquiringseismic data that includes: receiving seismic signals at a sensor;sampling the received seismic signals from the sensor into a pluralityof samples, each sample having a same number of bits (“bit length”);arranging the samples in a packet, wherein the total number of bitscorresponding to the samples represented in the packet is less than thenumber of samples represented in the packet times the bit length; andtransmitting the packet to a remote unit (also referred to as thecentral recording unit, controller or central control unit).

In another aspect, the method may further include: receiving the packetat the control units; decompressing the packet; and storing theinformation relating to the decompressed samples in a suitable recordingmedium.

In another aspect, the disclosure provides a method for acquiringseismic data that includes: receiving seismic signals at one or moresensors; sampling the received seismic signals into a plurality ofsamples, each sample having a selected bit length; arranging the samplesin a plurality of packets; transmitting the plurality of packets,wherein time interval between transmissions of the packets varies.

In another aspect, the disclosure provides an apparatus that includes: acircuit for receiving seismic signals from a sensor; a circuit forsampling the received signals; a circuit for digitizing the samples,each digitized sample having a bit length; and a processor that arrangesthe digitized samples into packets, wherein at least some of the packetsinclude one or more compressed samples. Alternatively or in addition tousing compressed samples, the processor may vary the time intervalbetween the transmissions of packets to reduce noise.

It should be understood that examples of the more important features ofthe apparatus and methods for acquiring and transmitting seismic datafrom the data acquisition units in a seismic spread have been summarizedrather broadly in order that detailed description thereof that followsmay be better understood and in order that the contributions to the artmay be appreciated. There are, of course, additional features of suchapparatus and methods that will be described hereinafter and will formthe subject of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

The various features described herein will be best understood from theattached drawings, taken along with the following description, in whichlike numerals generally have been used to represent similar elements,and in which:

FIG. 1 shows a cable seismic data acquisition system wherein the dataacquisition units may include a data compression and/or a time-slotvariance manager;

FIG. 2 shows a wireless seismic data acquisition system wherein the dataacquisition units may include data compression and/or a time-slotvariance manager;

FIG. 3 shows a high level functional block diagram of the dataacquisition units that may be utilized in the seismic data acquisitionsystems of FIGS. 1 and 2;

FIG. 4 shows a functional flow diagram for performing a data compressionaccording to one exemplary embodiment;

FIG. 5 shows a functional flow diagram for time-slot variance managementrelating to the transmission of packets according to one exemplaryembodiment;

FIG. 6 shows a functional flow diagram for data compression andtime-slot variance management of packets according to one exemplaryembodiment;

FIG. 7 shows an exemplary flow chart of a method for data compressionand time-slot variance management of packets;

FIG. 8 shows an exemplary packet that includes an epilog, a payload anda prolog, wherein the number of bits for each sample in the payload isthe same;

FIG. 9 shows an exemplary packet that includes an epilog, a payload anda prolog, wherein some of the samples in the payload are compressedwhile the other samples are not compressed; and

FIG. 10 shows another example of a packet that includes different setsof samples that are differently compressed.

DETAILED DESCRIPTION

The drawings shown and the descriptions provided herein correspond tocertain specific embodiments for the purposes of explanation of theconcepts contained in the disclosure herein with the understanding thatthe present disclosure is to be considered an exemplification of theconcepts and principles described herein and is not intended to limitthe scope of the claims relating to this disclosure.

FIG. 1 depicts an exemplary land cable seismic data acquisition system100, wherein certain elements such as the data acquisition units mayinclude some or all of the features described herein relating to thedata compression and/or time slot variance management and perform thevarious functions described herein. The system 100 is shown to includean array of spaced-apart seismic sensors or receivers 102, arrangedalong a number of lines or strings 108. Each line contains a number ofdata acquisition units or devices 103. Each data acquisition unit 103 ineach line 108 is coupled to a cross-line unit 104. Several cross-lineunits 104 and associated lines are usually coupled together by cabling,such as shown by the dotted line 114, which is then coupled to centralcontrol unit or control unit, such as a unit 106, which may be stationedon a mobile unit, such as a truck 120.

The sensors 102 are usually spaced several meters apart (for examplebetween 30-80 meters) and each line 108 may include several dataacquisition units 103 connected by communication lines 110. Each dataacquisition unit 103 typically includes a preamplifier that amplifiesthe signals received from its corresponding sensors 102, samples theamplified signals into a number of discrete digital representations(“samples”) having a fixed number of bytes, each byte containing a fixednumber of bits. The successive data acquisition units in a line act asrepeaters of data received from their respective preceding dataacquisition units. Each cross-line unit 104 may perform some signalprocessing and then store the processed signals as seismic informationfor later retrieval. The cross-line units 104 act as repeaters and aretypically coupled, either in parallel or in series, with one of theunits 104 a serving as an interface between the central control unit orcontrol unit (CU) 106 and a number of cross-line units 104. In the cablesystem of FIG. 1, data are usually relayed from one data acquisitionunit to the next unit and through several cross-line units before suchdata reaches the central control unit 106. The control unit is inbidirectional data communication with each data acquisition unit 103, asshown by data flow arrows 109. A source, controlled by the control unitis activated to induce seismic energy into the earth at selectedlocations in the seismic spread.

Referring to FIG. 2 there is shown a representation of a wirelessseismic data acquisition system 200 according to one embodiment whereinthe various data acquisition units may include the data compressionand/or time-slot variance features described herein. The system 200includes a central controller 202 in data communication with eachwireless data acquisition unit 208 forming an array (spread) 210 forseismic data acquisition. The data acquisition unit 208 and dataacquisition unit 103 of FIG. 1 may be configured to perform the samefunctions relating to the data compression and transmission of suchdata. The wireless communication between the central controller 202 withthe data acquisition unit 208 may be direct bidirectional wirelesscommunication or via an intermediate unit such as a repeater unit (RU).Each data acquisition unit 208 includes one or more sensors 212 forsensing seismic energy. The sensors 212 may be any suitable seismicsensors, including geophones, and one or more component accelerometers.Direct communication, as used herein, refers to individualized data flowas depicted in FIG. 2 by dashed arrows. The data flow is bidirectionalbetween the central controller 202 and the wireless data acquisitionunit 208. The communication might be in the form of radio signalstransmitted from and received by the data acquisition units 208 andcentral controller 202 via suitable antennas 203 and 204 respectively.

In one aspect, a seismic energy source 206, such as an explosive source,a vibrator carried by a mobile unit, such as a truck 202, or acompressed gas source, generates seismic energy of knowncharacteristics, such as magnitude, frequency, etc., at known locationsin the seismic spread to impart seismic energy into the subterraneanformation. The source controller 274 can be programmed to receive andtransmit information such as instructions to make the source 206 i readyfor firing, fire the source 206 i, provide data indicative of thelocation of the mobile unit 270, the arming status of the source 206 i,and data such as return shot attributes.

The functions described above in reference to FIG. 2 that relate to theoperation and control of the source and those of the control unitequally apply to the cable seismic spread of FIG. 1.

In another aspect, the seismic spread configuration shown in FIG. 2 maybe modified, wherein a number of neighboring data acquisition units 208forming a “group” or “cell” communicate within the control unit 202 viaan intermediate data acquisition unit (also referred to herein as anAlpha unit). An Alpha unit may also be configured to perform thefunctions of the data acquisition unit and further configured toperformed a variety of other functions, such as establishing two-waycommunication between the Alpha unit and its associated data acquisitionunits. In this manner, the various data acquisition units may be groupedinto several groups, each group including an Alpha unit. For example,the data acquisition 220 in the group 222 may be an Alpha unit for thegroup of data acquisition units in the geographical area 222. Othergroups of data acquisition units in the seismic spread 210 may besimilarly grouped.

Alternatively, one or more separate repeater units (RUs) may be placedat selected locations in the seismic spread 210, such as shown byrepeaters R₁, R₂ . . . R_(n), etc. Often only one repeater is used in aseismic spread. Each repeater unit may be configured to establish atwo-way radio or wireless communication between its associated dataacquisition units and the control unit 220. In the above-notedconfigurations, the individual data acquisition units communicate withtheir associated Alpha unit or the repeater unit as the case may be andthe Alpha unit or the repeater unit communicates with the centralcontroller 202. The individual data acquisition units in a groupwirelessly communicate with their associated Alpha unit or the repeaterunit wirelessly. In certain situations, it may be desirable to connectthe data acquisition units to its associated Alpha unit with electric orfiber optic.

FIG. 3 shows a high level functional block diagram 300 of the dataacquisition units that include circuitry and perform functions andmethods according to the various aspects of the disclosure. Each dataacquisition unit 302 is shown coupled to one or more seismic receivers102 from which it receives signals generated by the receivers inresponse to the detection of seismic signals from the earth'ssubsurface. The detected signals may be in response to the activation ofa seismic source as described above and/or the seismic signals producedin response to noise, such as generated by vehicles passing near thesurvey area, thunderstorms, rails, ships, etc. Each data acquisitionunit 302 is shown to include a signal acquisition section 310 thatcontains circuitry for receiving signals from the sensor 102, circuitryfor conditioning the received signals and circuitry for sampling thereceived signals. Each signal acquisition section 310 may include aprocessor 320, a memory 322 that may store data and computer programs324, which programs are executed by the processor 320 to perform thevarious operations and methods described herein and to perform otherfunctions known in the art for such devices.

The signal acquisition section 310 samples the received signals in smalltime units. As an example, the control unit 350 may send a commandsignal to a particular data acquisition unit 302 a to start acquiringdata. The command, for example, may be sent prior to the firing of theseismic source. The data acquisition unit 302 a then starts to receivethe signals from its associated sensors 102, conditions the signals andthen samples the signals conditioned. As an example, the dataacquisition unit 302 a may be configured to acquire data for a selectedtime, for example three seconds, with a selected sample time such as twomilliseconds. In this example, the data acquisition unit 302 a willstart sampling a particular signal from a receiver 102 at time to everytwo milliseconds to provide 1500 samples. The data acquisition unit 302a may include a digitizer 312 that digitizes each sample, wherein eachsample has the same number of bytes. For example, each sample may havethree bytes, each byte having eight bits. In this example, eachdigitized sample (also referred to herein as a “word”) will havetwenty-four bits. The samples may contain any other suitable number ofbits, for example 12, 16, 32 or more. The processor 320 in the dataacquisition unit 302, utilizing the programmed instructions 324, storesthe digitized samples in the memory 322, which may be a buffer. In oneaspect, the processor stores all the bits for the samples. In anotheraspect, the processor 320 may delete a selected number of leading signbits from selected digitized samples and store such compressed samplesin the memory. In such a case, the compressor may insert an indicatorcorresponding to the compressed samples which indicator will enable theprocessor to arrange the compressed samples into packets and also enablethe decompression of the compressed samples at a later time, for exampleusing the control unit. The indicator may correspond to any particularstored sample or a group (series) of samples. The stored digitizedsamples are then arranged in packets by the processor 320 as describedbelow with respect to FIGS. 7, 9 and 10.

Still referring to FIG. 3, a data transmission and repeater section 360transmits the packets via the communication link 330 to the next dataacquisition unit, such as unit 302 b, which transmits the packetsreceived from its preceding data acquisition units and the packetscreated by the unit 302 b itself to the next data acquisition unit orthe data repeater, such as the data repeater 370, at the end of theline. Each repeater 370 may transmit the received packets to one or moremedia converters 380. The media converters 380 may be serially coupledso that the last such media converter 380 a transmits the data receivedfrom other media converters and the repeaters from its correspondingline to the control unit 350 as well as to data archive 352 ordecompressor 354. Thus, in the configuration of FIG. 3, the dataacquisition units 302 acquire, condition, sample, and digitize samplesand transmit the digitized data to the control unit 350 in the form ofpackets. The communication links 330 in FIG. 3 may be electricconductors, fiber optic links or wireless connections. Also, the dataacquisition units 302 may transmit the packets directly to a commonrepeater associated with a group of data acquisition units, which inturn sends the packets to the control unit 350.

In one aspect, the data acquisition units 302 may include a datacompressor 314. The data compressor may be a program or routine that mayinclude any suitable data compression algorithm. The data compressionprogram and algorithm are stored in the memory 322, which program andalgorithm are utilized by the processor 320 to compress the digitizedsample data prior to incorporating such samples into a packet.

FIG. 4 shows a configuration wherein an exemplary data acquisition unit402 acquires seismic signals and digitizes the seismic signals insection 404. A data compressor 406 compresses the digitized samples, andforms the packets, which are transmitted by the data telemetry andrepeater section 408 to the control unit 350 (FIG. 3).

FIGS. 8-10 show examples of data packets that may be made according toone aspect of the disclosure. FIG. 8 shows a data packet 800 having anepilog 802, a payload or seismic data section 804 and a prolog 808. Inone aspect, the epilog 802 may contain a fixed number of bytes thatcontain information relating to a destination address, source addressand other information that enables the control unit 350 to appropriatelydecipher the received packets and relate the particular samples toparticular sensors in the seismic spread. The prolog 808 may containpacket confirmation information or other desired information. Thepayload section typically may include bit spaces for “n” samples(designated as samples n₁ though n_(n)), each sample having the samenumber of bits “p.” Thus, the payload may contain n×p total bits. In theexample of FIG. 8, each sample occupies “p” bits, shown by the segment810. During seismic data acquisition, a sequence of samples may be suchthat the digitized words occupy most of the “p” bits, i.e., without asignificant number of leading sign bits. Such are typically arranged insample packets without data compression.

FIG. 9 shows an example of a packet in which the sample has beencompressed by removing a certain number of leading sign bits from acertain number of samples. In the example of FIG. 9, the payload section904 is shown divided into two sections 910 and 912, wherein the section910 includes “n” samples (m₁ through m_(n)), each such sample having thefull word length, i.e., “p bits,” while the section 912 has “q” samplesm_(n+1) through m_(n+q), each such sample having “p-z” bits, where z isa whole number. Thus, if p=12 and z=4, the samples m₁ through m_(n) eachwill occupy 12 bit spaces while the remaining samples m₊₁ throughm_(n+q), each will occupy 8 bit spaces. Such a method enables using thespaces vacated due to the removal of leading sign bits to packadditional samples in the same packet. For example, if the capacity of apacket is 500 samples of twenty-four bits, and 300 of the samples havefour leading sign bits removed, this will open up 300×4=1200 bit spacesfor packing additional samples in that packet. In order for the controlunit to decompress the packet data, the data acquisition unit processoris programmed to encode the packet that contains the compressed data. Inone aspect, the processor may insert an instruction word or a leadingindicator that precedes the set or sequence of samples that will havethe leading sign bits removed. In one aspect, the instruction word orthe leading indicator simply may occupy a desired number of bits, suchas shown by bit spaces 922, containing the instructions that mayindicate the number of compressed samples following the indicator andthe nature of compression, i.e., the number of bits removed from eachsuch compressed sample. Any suitable instruction may be used as theinstruction word or the leading indicator. If the digitized samples arestored in a memory in compressed form, then the processor may arrangesuch compressed samples in a packet.

FIG. 10 shows another example of a packet wherein the first section 1010of the payload section 1002 includes a first leading indicator 1012 thatcorresponds to a certain number of samples, each having “p-a” bits and asecond section 1020 that includes a second leading indicator 1022 thatcorresponds to a certain number of samples, each having “p-b” bits. Ineach case, the leading indicator may include information that enablesthe control unit to determine the number of compressed samples and thenumber of bits present in each sample or the number of leading sign bitsthat have been removed from each such sample. The control unit receivesthe packets from the various data acquisition units, decompresses thepackets and stores the samples. Each packet may include any combinationof compressed and uncompressed samples, arranged in any desired order.The control unit may process the samples and may store the processedsamples and/or transmit the processed data to a remote processing unitfor further processing to obtain the subsurface maps. The packets maycontain any number of combinations of uncompressed and compressedsamples.

In another aspect, the data acquisition units may perform time variancerelating to the transmissions of packets to reduce the coherent noisethat is often associated in the seismic data recording units. In oneaspect, the time variance may be performed after the packets have beenprepared by the data acquisition unit, such as shown in FIG. 5. In oneaspect, a time variance manager 510 associated with the data acquisitionunit 500 may include a routine or algorithm that is used by theprocessor in the data acquisition unit to vary the time between thetransmissions of successive packages. In one aspect, the time variancemanager may utilize a suitable random number generator to determine thetime between transmissions of successive packets. Varying the timebetween the transmissions of the packets makes the coherent noise toappear at non-periodic times, which the seismic data acquisition systemcan distinguish from the seismic signals and is thus able to remove suchnoise from the seismic signals. When the time between the transmissionsof packets is constant, a periodic noise, such as a coherent noise, canappear to the system as a seismic signal, thereby providing erroneousmeasurements. The time variance manager also may utilize time slots(bins) and a random time variance within each time slot to determine thetime variance between the transmissions of the packets.

In another aspect, the data acquisition unit 600 may perform thetime-slot variance after the data compression, as shown in FIG. 6. Inthe configuration of FIG. 6, the data compressor 604 compresses the dataas described above or by any other suitable method and assembles thepackets. The time slot variance manager 610 then varies the time betweentransmissions of successive packets. The data telemetry section 612 thentransmits the packets to the central unit. Because seismic signals havean inherent signal variance from trace to trace, the number ofcompressed samples can be different, therefore the time it takes thedata acquisition unit to prepare successive packets can vary, which cancause the time between transmissions of packets to vary, therebyreducing the coherent noise.

FIG. 7 shows an exemplary flow diagram 700 of a method that may beutilized by the data acquisition units to prepare and transmit datapackets. The data acquisition unit upon receiving a command from thecentral control unit starts the process of acquiring samples from itsassociated sensors, such as sample “x,” as shown in box 712 andcompresses the digitized sample “x” to form the sample “z” having aselected bit length and stores the digitized sample in the memory, asshown at box 714. The processor then computes the packet efficiency “PE”(box 716), which may be defined as the number of samples represented inthe packet. The PE depends upon the number of total leading sign bitsdeleted from all the samples in a packet. The processor forms the packetwith appropriate epilog, payload and prolog and appropriate leadingindicators, such as 1012 and 1022 of FIG. 10. The processor thendetermines whether the packet is full as shown in the decision box 718.A full packet means that all bit spaces in the payload section of thepacket have been used or occupied. If “no,” i.e., the packet is not full(decision line 721), the processor waits for the next sample (box 742)and acquires the next seismic sample “x” (box 712) and continues theprocess of acquiring and compressing samples (when appropriate) tocomplete the packet. When the packet is full (decision line 719), theprocessor may transmit the packet to the control unit. Alternatively,the processor may determine whether the PE is less than a presetthreshold (box 720), which may be any suitable value provided to theprocessor, either as a stored value in an associated memory or via acommand from the control unit.

In one aspect, when the PE is less than the threshold (as shown bydecision line 722), the processor transmits the packet to the controlunit without applying any time variance technique, as shown in block724. When the PE is equal to or greater than the threshold, as shown bydecision line 726, the processor may be programmed to compute a variancetime slot for the packet, as shown at box 728 and wait for the time slot(box 730) and thereafter transmit the packet as shown at box 724. Once aparticular packet is transmitted, the processor determines if the recordfor a seismic signal has been completed (box 732), i.e., all the packetscorresponding to a particular record have been transmitted. If “yes”(decision line 738), the process waits for the start of the next record,as shown in block 740, sets the record criteria (box 741)for the nextrecord and starts the process of acquiring samples as provided above. Ifthe record has not yet been completed (decision line 736), the processorwaits for next sample (box 742) and continues to acquire seismic samplesto form the next packet. The above-described flow chart shows oneparticular method. However, any flow scheme that provides forcompressing the data in the manner described herein and/or uses a timevariance technique may be utilized for the purposes of this disclosure.It should be noted that the methods and functions described hereinequally apply to data acquisition units contained in marine seismic dataacquisition systems.

Thus, the disclosure herein in one aspect provides a method of acquiringseismic data that includes: receiving seismic signals at a sensor;sampling the received seismic signals from the sensor into a pluralityof samples, each sample having a same number of bits (bit length);arranging the samples in a packet, wherein the total number of bitscorresponding to the samples represented in the packet is less than thenumber of samples represented in the packet times the bit length of thesamples represented in the packet; and transmitting the packet to aremote unit. In one aspect, certain number of leading sign bits from atleast some of the samples may be removed before arranging such samplesin the packet, thereby compressing the data corresponding to the samplesrepresented in the packet. The leading sign bits may be the leadingzeros or leading ones. The method may further include inserting anindicator in the packet that identifies the samples in the packet thathave bits removed therefrom. The method may further include transmittingthe packet to a control unit. The packet may be transmitted by anysuitable manner, including via a land cable, wirelessly, an ocean-bottomcable, or a streamer cable that is in data communication with thecontrol unit on a vessel, and electrical conductor or fiber optic links.In one aspect, the samples may be arranged such that each samplebelonging to one set of samples in the packet occupies the number ofbits that is equal to the bit length and each sample belonging to asecond set occupies the number of bits less than the bit length. Anycombination of compressed and uncompressed samples may be used in aparticular packet.

In another aspect, the method may include receiving a packet at theremote unit; decompressing the packet; and storing the informationrelating to the samples in the packet in a suitable medium, such assolid state memory, hard disc, tape, etc. The received samples in oneaspect may be in response to the transmission of a seismic signal intothe earth and in another aspect generated by noise. In another aspect, amethod is provided that includes: transmitting an acoustic signal intothe earth; receiving seismic signals at a one or more sensors in signalcommunication with the earth; sampling the received seismic signals fromthe one or more sensors into a plurality of samples and digitizing eachsample at a data acquisition unit in signal communication with the oneor more sensors, each digitized sample having a predefined bit length;arranging the digitized samples into a plurality of packets, wherein atleast some of the packets in the plurality of packets include digitizedsamples are compressed samples so that each compressed sample occupiesless than a bit length; and transmitting the plurality of packets to aremote unit. The transmitting of the packets may include transmittingthe plurality of packets with a varying time intervals between thetransmissions of at least some of the packets. The time intervals may becomputed using any technique that randomizes the time intervals,including a random number generator. The method may compute packetefficiency for each packet before computing the time intervals betweenthe transmissions of the packets and may transmit the packets withoutvarying the time intervals when the packet efficiency is less than acertain threshold. The varying time may be computed using a time slottechnique. The method provides for choosing a random time-slot for eachpacket across a portion or the entire seismic data acquisition systemelements so that no transmission device is on the same time schedule.The method provides for inserting an indicator in the packets thatincludes compressed digitized samples, which indicator will enable theremote unit to decompress the compressed digitized samples. The controlunit may receive the plurality of packets; decompress the compresseddigitized samples; process the decompressed samples that may includesuch techniques as stacking, correlating, noise editing, etc.; and storethe processed samples in a data storage medium. The processed samplesmay be used to obtain a map of the earth's subsurface.

In another aspect, the disclosure provides a method for acquiringseismic data that includes receiving seismic signals at one or moresensors; amplifying the received seismic signals into a plurality ofsamples, each sample having a fixed bit length; arranging the samples ina plurality of packets; transmitting the plurality of packets, whereinthe time interval between the transmissions of successive packetsvaries. As noted earlier, the time interval between the transmissions ofthe successive packets may be computed using any technique thatrandomizes the time intervals, including a random number generatorand/or using a time-slot technique.

In another aspect, the method may form packets having different payloadsizes, wherein some packets may include compressed samples and maytransmit such packets with or without varying the time intervals betweenthe transmissions of such packets.

In another aspect, the disclosure provides a seismic data acquisitionapparatus that includes: a circuit for receiving seismic signals from asensor; a circuit for sampling the received signals; a circuit fordigitizing the samples, each digitized sample having a fixed bit length;and a processor that arranges the digitized samples into packets,wherein at least some of the packets include at least some of thesamples that occupy number of bits less than the fixed bit length. Thedata acquisition unit may further include a transmitter that transmitsthe packets over a communication link, which may be: (i) an electricalconductor; (ii) a wireless link; (iii) a data communication link in astreamer cable; (iv) a data communication link in an ocean-bottom cable;or (v) a fiber optic link. The processor may compress the samples byremoving certain leading sign bits from the samples. The processor maystore the compressed bit in a memory and then utilize such storedsamples to form packets. The processor further may insert an indicatorcorresponding to any particular compressed sample or a group of samplesso as to enable the decompression of the compressed samples at a latertime. A control unit placed remote from the sensors receives thetransmitted packets; and decompresses the compressed packets. Theprocessor also may vary the time interval between the transmissions ofsuccessive packets. A program associated with the processor enables theprocessor to vary the time based on: any suitable method including butnot limited to: (i) using a random number generator; and (ii) using atime slot computation for the packets.

Both the method of compressing data by deleting bits in samples andvarying time between transmissions of packets provide randomization.These methods may be utilized separately or concurrently.

The foregoing description is directed to particular embodiments for thepurpose of illustration and explanation. It will be apparent, however,to one skilled in the art that many modifications and changes to theembodiments set forth above are possible without departing from thescope and the spirit of the disclosure. It is intended that thefollowing claims be interpreted to embrace all such modifications andchanges.

1. A method of acquiring seismic data, comprising: receiving seismicsignals at one or more sensors; sampling the received seismic signalsinto a plurality of samples, each sample having a same bit length;compressing at least some of the samples in selected packets beforearranging such compressed samples into packets; arranging the samplesinto a plurality of packets; computing packet efficiency for a packet ofthe plurality of packets before computing the time intervals between thetransmissions of the packets; and transmitting the plurality of packetsby varying time intervals between transmissions of the packets.
 2. Themethod of claim 1 further comprising determining the time intervalsbetween the transmissions of the packets using a random numbergenerator.
 3. The method of claim 1 further comprising using a time slotscheme for assigning the time intervals between the transmissions of thepackets.
 4. The method of claim 1, wherein the time intervals betweenthe transmissions of the packets corresponds to a time slot assignmentscheme.
 5. The method of claim 1, wherein compressing samples comprisesdeleting selected bits from the at least some of the samples.
 6. Themethod of claim 1 further comprising placing an indicator thatidentifies the compressed samples in the packets.
 7. The method of claim1, wherein transmitting the plurality of packets further comprisestransmitting the packets via one of: (i) land cable; (ii) wirelessly;(iii) an ocean bottom cable; and (iv) a streamer cable that is in datacommunication with a central recording unit on a vessel.
 8. The methodof claim 1 further comprising receiving the transmitted packets at aremote unit and decompressing compressed packets.
 9. The method of claim8 further comprising processing the decompressed packets to obtaininformation about a subsurface structure.
 10. The method of claim 1further comprising varying the time intervals between transmissions ofthe packets in a manner that reduces coherent noise in the transmissionof the packets.
 11. A seismic data acquisition apparatus, comprising: acircuit configured to receive seismic signals from a sensor and digitizethe received seismic signals to provide a plurality of samples, eachsample having a selected bit length; a processor configured to: compressat least some of the samples in selected packets before arranging suchcompressed samples into packets; arrange the plurality of samples into aplurality of packets; compute packet efficiency for a packet of theplurality of packets before computing the time intervals between thetransmissions of the packets; and transmit the plurality of packets byvarying time intervals between the transmissions of the plurality ofpackets.
 12. The seismic data acquisition apparatus of claim 11, whereinthe processor is further configured to randomize the time intervalsbetween the transmissions of the packets.
 13. The seismic dataacquisition apparatus of claim 11, wherein the processor is furtherconfigured to utilize a time slot scheme for the transmission of thepackets.
 14. The seismic data acquisition apparatus of claim 11, whereinthe processor compresses the selected samples by deleting selected bitsfrom such samples.
 15. The seismic data acquisition apparatus of claim11, wherein the processor is further configured to place an indicator inthe packets that include compressed samples in order to identify thecompressed samples.
 16. The seismic data acquisition apparatus of claim11 further comprising a remote unit in data communication with theprocessor, wherein the remote unit is configured to receive thetransmitted packets; decompress the received packets; and store thedecompressed packets.
 17. The seismic data acquisition apparatus ofclaim 11, wherein the packets are transmitted via one of: (i) a landcable; (ii) wirelessly; (iii) an ocean bottom cable; and (iv) a streamercable in data communication with a central recording unit on a vessel.18. The seismic data acquisition apparatus of claim 11, wherein theprocessor is further configured to vary the time intervals betweentransmissions to reduce coherent noise.
 19. A method of acquiringseismic data, comprising: receiving seismic signals at one or moresensors; sampling the received seismic signals into a plurality ofsamples, each sample having a same bit length; compressing at least someof the samples in selected packets before arranging such compressedsamples into packets; placing an indicator that identifies at least oneof: (i) a number of compressed samples, (ii) a number of bits in eachsample, and (iii) a number of bits removed from each sample; arrangingthe samples into a plurality of packets; computing packet efficiency fora packet in the plurality of packets before computing the time intervalsbetween the transmissions of the packets; and transmitting the pluralityof packets by varying time intervals between transmissions of thepackets.
 20. The method of claim 19 further comprising using a time slotscheme for assigning the time intervals between the transmissions of thepackets.
 21. The method of claim 19 further comprising determining thetime intervals between the transmissions of the packets using a randomnumber generator.
 22. The method of claim 19 further comprising varyingthe time intervals between transmissions of the packets in a manner thatreduces coherent noise in the transmission of the packets.